(1) Field of the Invention
This invention relates to a capacitor for semiconductor storage device and a process for fabricating the capacitor.
(2) Description of the Prior Art
In order to realize dynamic RAMs of higher integration, a variety of measures have been adopted against soft errors. According to one of the measures, the capacitors constituting a dynamic RAM are formed in three-dimensional structure to increase the capacitor surface area so that each capacitor can store an amount of signal charge with sufficient tolerance for soft-error noises. In the capacitor of such structure, however, the storage capacitance portion is formed in a steeply rugged area and, therefore, it is necessary that a capacitive dielectric film for forming the storage capacitance portion should have a very good step coverage property. From this point of view, a film of tantalum oxide (Ta.sub.2 O.sub.5), which is a highly dielectric material, has been deposited by a chemical vapor deposition (CVD) process in order to form storage capacitance portions with good step coverage. The tantalum oxide film is formed by use of a reactant gas containing a metal-organic compound which shows rapid surface migration, such as methoxytantalum [Ta(OCH.sub.3).sub.5 ] and ethoxytantalum [Ta(OCH.sub.2 H.sub.5).sub.5 ].
Besides, in the recent semiconductor storage devices, there is a trend toward a lower power-supply voltage. A loss in charge storage capacity due to the lowered supply voltage is often compensated for by a reduction in the material thickness of storage capacitance portions.
Alternatively, a method of treating the storage capacitance portions with an ammonia (NH.sub.3) plasma to introduce nitrogen into the storage capacitance portions is practiced, in order to compensate for the loss in charge storage capacity.
However, the capacitor as described above has the drawback that a reduction in the material thickness of the storage capacitance portion causes an increase in leakage current, leading to a lowered charge storage capability. Therefore, dynamic RAMs fabricated with such capacitors used to constitute storage capacitors are low in data storage performance.
To overcome this problem, a structure has been proposed in which the storage capacitance portions are formed by stacking a silicon oxide (SiO.sub.2) film and a silicon nitride (Si.sub.3 N.sub.4) film, thereby reducing leakage current. Even in the case of capacitors with this structure, if the material thickness of the storage capacitance portion is reduced to or below 50 nm, the dielectric strength is lowered greatly and leakage of currents occurs more frequently.
Furthermore, according to the method of introducing nitrogen into the storage capacitance portions by an ammonia (NH.sub.3) plasma treatment, it is difficult to introduce a sufficient amount of nitrogen. Therefore, it is impossible to suppress satisfactorily the leakage of currents.